34 STRUCTURE OF MINERALS. 



of a crystal, being obtained with equal ease, and affording 

 planes of like luster : and conversely, it is dissimilar paral- 

 lel to dissimilar planes. It is accordingly the same, parallel 

 to all the faces of a cube ; but in the square prism, the basal 

 cleavage differs from the lateral, because the base is unequal 

 to the lateral planes. Often there is an easy cleavage par- 

 allel to the base, and none distinct parallel to the sides, as 

 in topaz ; and so the reverse may be true. 



The thirteen fundamental forms enumerated, are the solids 

 obtained from the various minerals by cleavage. 



Some minerals present peculiar cleavages of a subordinate 

 character, independent of the principal cleavage. Calc spar, 

 for example, has sometimes a cleavage parallel to the longer 

 diagonal of its faces. The facts on this subject are of con- 

 siderable interest, yet not of sufficient importance to be dwelt 

 on in this place. 



SECONDARY FORMS. 



If crystals always assumed the shape of the primary form, 

 there would be comparatively little of that variety and beauty 

 which we actually find in the mineral kingdom. Nature 

 first taught to heighten the brilliancy of the gem by covering 

 its surface with facets. To the uninstructed eye, these cubes 

 and prisms with their numberless brilliant surfaces, often 

 appear as if they had been cut and polished by the lapidary : 

 yet the skill and finish of the work, most perfect in the 

 microscopic crystal, has but feeble imitation in art. Not 

 unfrequently, crystals are found with one or two hundred dis- 

 tinct planes, and occasionally even a much larger number ; 

 and every edge and angle has the utmost perfection, and the 

 surfaces an evenness of polish, that betrays no rude work, 

 manship, even under the highest magnifying glass. Cavities 

 are occasionally met with in the rocks, studded on every 

 side with crystals — a crystal grotto in minature — sparkling 

 when brought out to the sun like a casket of jewels. Even 

 amid the apparent confusion, there is wonderful order of 

 arrangement in the crystals : the corresponding planes gen- 

 erally face the same way, so that the sparkling effect appears 

 in successive flashes over the surface, as every new set of 

 facets comes in turn to the light. Add to this view, their 

 delicate colors — the rich purple of the amethyst, the soft 

 yellowish shades of the topaz, the deep green of the erne. 



On what does the beauty of crystals to a great extent depend ? 



